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AN980 Datasheet(PDF) 4 Page - STMicroelectronics |
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AN980 Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 8 page 4/8 ST7 KEYPAD DECODING TECHNIQUES, IMPLEMENTING WAKE-UP ON KEYSTROKE 3 EXTERNAL INTERRUPTS The MCU is in HALT mode. When a key is pressed, a low level is applied to the pin corre- sponding to the row the key belongs (pins configured as inputs with pull-up). It’s a falling edge applied to a pin of Port C which creates an external interrupt (EI1) and wakes up the MCU. The MCU executes then the external interrupt subroutine (decoding the pressed key) and comes back to its previous state (HALT state in the main program). 4 KEYPAD The keypad used is a 4x4 matrixed keypad. Rows are connected to pins configured as inputs with pull-up. So the initial state of these pins are a high level (1). When a key is pressed, a low level is applied to the corresponding pin. For this reason, the keypad is coded as follows: Table 1. Key values KEY row value column value KEY row value column value 1 0x0E 0x0E 7 0x0B 0x0E 2 0x0E 0x0D 8 0x0B 0x0D 3 0x0E 0x0B 9 0x0B 0x0B F 0x0E 0x07 D 0x0B 0x07 4 0x0D 0x0E A 0x07 0x0E 5 0x0D 0x0D 0 0x07 0x0D 6 0x0D 0x0B B 0x07 0x0B E 0x0D 0x07 C 0x07 0x07 12 3 F 45 6 E 7 8 9 D A 0 B C PC0 PC1 PC2 PC3 PA0 PA1 PA2 PA3 |
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